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Spider silk is five times stronger than steel—now, scientists know why

The next time you brush aside a spiderweb, you might want to meditate on its delicate strength—if human-size, it would be tough enough to snag a jetliner. Now, scientists know just how these silken strands get their power: through thousands of even smaller strands that stick together to form this critter’s clingy trap.

To find out how most spider silk is five times stronger than steel, scientists analyzed the silk that venomous brown recluse spiders use to create their ground webs and hold their eggs, using an atomic force microscope. They found that each strand—which is 1000 times thinner than a human hair—is actually made up of thousands of nanostrands, only 20 millionths of a millimeter in diameter, they reported last month in ACS Macro Letters. Just like a tiny cable, each silk fiber is entirely composed of parallel nanostrands, which they measured to be at least 1 micron long. That may not sound very lengthy, but on a nanoscale, it’s at least 50 times as long as these fibers are wide—and researchers believe they could stretch even further.

The idea that nanofibers make up spider silk has been proposed before, but until now, there was no evidence to suggest nanostrands comprised the entire makeup of a silk fiber. The team’s secret weapon was the unique silk of the brown recluse spider, which, unlike most, is a flat ribbon as opposed to a cylindrical fiber, making it easier to examine under the lens of a powerful microscope.

The new discovery builds on a finding the team made last year, which demonstrated how the brown recluse spider reinforces its main silk strands with a special looping technique (above). Equipped with a tiny sewing machine–like spinneret, the spider weaves about 20 microloops into every millimeter of silk it ejects, which strengthens their sticky spool and prevents it from collapsing.

Researchers say even though the flat ribbons and looping technique are not shared by all spiders, their study of brown recluse silk may be a window to exploring the stringy fibers of other species. Such studies could pave the way for creating new materials that could be used in medicine and engineering. But synthetic spider silk has been notoriously difficult to create. In the meantime, researchers hope their work will help us unreel one of the toughest materials of the natural world.